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tech question? bridging QSC amp - one cabinet works, the other doesn't - no idea why

Discussion in 'Amps and Cabs [BG]' started by patrickj, Jan 18, 2006.

  1. patrickj


    Aug 13, 2001
    Baltimore, MD
    Both of these are cabinets that I built. The bridged cable is banana plug -> speakon (for the cabinets). Only two leads so the speakon side is wired 1+ and 1-. The cable has passed with a voltmeter.

    The first cabinet (1x18 + 2x10) is roughly 6.4 ohm. It works with the QSC amp in both normal and bridged mode with the banana cable.

    The second cabinet (4x12 + 2x10) is also roughly 6.4 or 10.6 ohm (I have to double check which wiring scheme I used - I don't remember off hand) and doesn't work with the banana cable in either normal or bridged mode. The amp powers up shows a signal on the LEDs, but no sound comes through the speakers (not even at low volume).

    The weird thing though is that the same cabinet works fine with any different (non bridged or bridged intended) cable. If I use a straight Speakon->Speakon cable, it works perfectly in normal mode.

    What's going on? I don't think there's anything wrong with this PA (it works for absolutely everything else I've plugged into it in normal and bridged) and also because I have 2 QSC amps and both do the same thing (consistency - so I'm looking at the cabs). I'm blaming the wiring somewhere but I don't know where. I can't figure out a case where one cab works, another one doesn't - both with the same cable (eliminates the cable being the problem). But then, both cabs work with a different cable (eliminates the cabinet being the problem).

    Is the impedance to high on a 10.6 ohm cabinet? Even if this is the case and explains why no signal at all comes through with the banana cable, why does it then work with a stock Speakon cable (has 4 leads - is this it? - on paper, I can't see why it would make a difference).

    Bob Lee? Anyone? Aieee!!?
  2. patrickj


    Aug 13, 2001
    Baltimore, MD
    Figures - as soon as I post a whole thread, thinking logically about the whole problem, 20 minutes later I realize the answer immediately as soon as I pull off the jack plate.

    I wired up the jack on the cabinet incorrectly. Rather than 1+ 1-, I wired it 1+ 2- (uhh?). The 4 lead cable was making a fake ground due to the extra leads so it worked-ish. It would also explain why (der) the cable worked elsewhere on other gear.